My rear wheel has a 135 mm Surly New Hub (flip flop variety), and I've noticed that there is some play in the hub: if I grab the wheel by the tire/rim and wiggle it side to side a bit I can clearly feel that there is some slop.

I expect that once I get out of bed this morning and get the wheel off the bike I'll see how to adjust it (is this indeed straightforward?), but more importaintly, is it likely that any damage has been done to the hub/bearings during the time that I've ridden the bike with the hub this way?

Thanks in advance if anyone has experience with this!

you probably havent done any damage. to adjust it you'll need 2 cone wrenches, and 18 and a 15mm i believe....

yup, that'll do it. my front hub did the same on a couple rides, but I got her tightened properly now.

I recommend replacing the outside locknuts (the 18mm ones) with smaller nuts, ones that don't have the high flange. The ones that come with the hub tend to snap. Then it's just a matter of keeping them tight...

Call Surly about this. They are very helpful with their explanations. There may be directions on their website too. Also, one great thing about those hubs is that they have no proprietary parts. Anything that breaks can be obtained easily from a standard bike supply co. I got one of the track hubs back before I knew much about bike maintinence and really abused it. The lock nuts broke, the bearings got smashed, and the axle stripped. I overhauled it for about $30 and it is good as new.

Thanks for the help everyone. It was indeed very simple once I had the correct cone wrenches. Just for the record: that outer locknut is a 17 mm, not an 18 mm (biggest I had in my collection was a 16 so I had to get a new one either way).

Just for completeness....

https://www.parktool.com/repair_help/howfix_hub.shtml